1. Technical Field of the Invention
The present invention relates to a method for producing an organic insulating coating such as a protective coating for electrodes in ink chambers of an ink-jet printhead, and further to an ink-jet printhead produced according to the method.
2. Description of Related Art
In place of impact printers, there has been a rapid diffusion of non-impact printers suitable for color and multiple-tone printing such as ink-jet printers. In particular drop-on-demand printers, which eject ink droplets only when needed to print on media, are popular because of their improved printing efficiency and low production and running costs. Most of the drop-on-demand printers today are using a Kyser method utilizing piezoelectric elements or a thermal ink-jet method.
In the Kyser-type printers, however, printheads are difficult to miniaturize and nozzle density thereof is difficult to increase. In the thermal ink-jet printers, although a high nozzle density is obtainable, since the energy of bubbles produced in ink by heating the ink with a heater is used to eject ink droplets, high ink durability is required, long life of the heater is hard to obtain, and power consumption is high.
To solve the foregoing problems, there has been proposed an ink-jet method according to which shear mode deformation of a piezoelectric material is utilized to eject ink. More specifically, an electric field perpendicular to a poling field of the piezoelectric material is applied to electrodes provided on sidewalls of an ink chamber made of the piezoelectric material to deform the sidewalls in shear mode, so that a pressure wave generated by the deformation is utilized to eject ink droplets through nozzle orifices. This method can realize a higher nozzle density, lower power consumption, and a higher drive frequency.
Illustrated in FIG. 11 is a configuration of a shear mode ink-jet printhead. The ink-jet printhead includes a base member 1 made of a piezoelectric material that is poled in the vertical direction to the plane of the drawing, with a plurality of grooves 4 formed on an upper surface thereof, a cover member 2 with an ink feed opening and a common ink chamber 22 provided, and a nozzle plate 9 with nozzle orifices 10. The grooves 4 in the base member 1 are formed into ink chambers 16 by attaching the cover member 2 and the nozzle plate 9 respectively to the upper surface and a lower surface of the base member 1. The ink chambers 16 are separated by sidewalls 3 having electrodes 5 on upper halves of the surfaces thereof for creating an electric field. Formed on the surfaces of the electrodes 5 are insulating coatings, or protective coatings, (not shown) for preventing the electrodes 5 from contacting ink filled in the ink chambers 16 directly.
Rear bottom edges of the ink chambers 16 are formed into an arc of a circle having radius of a dicing blade used to cut the grooves 4. The dicing blade is used to cut shallow grooves 6 as electrode lead parts for electrical conduction with the exterior. The electrodes 5 in the shallow grooves 6 are connected to external electrodes 8, for example on a flexible substrate, at rear ends of the shallow grooves 6.
Used as an insulating coating for preventing the electrodes 5 from contacting the ink is a poly-p-xylylene (known as parylene: “parylene” is a trademark of Nihon Parylene Kabushikikaisha) coating. The poly-p-xylylene coating is made from di-p-xylylene by CVD (chemical vapor deposition) method. Specifically, di-p-xylylene dimer is vaporized and then pyrolyzed to form stable monomeric diradical p-xylylene. The monomer simultaneously absorbs and polymerizes on a substrate to form a high-molecular-weight thin film. Hereinafter referred to as parylene N or poly-p-xylylene is the reaction product of di-p-xylylene dimer as dimeric p-xylylene, and referred to as parylene C or poly-monochloro-p-xylylene is the reaction product of di-p-xylylene dimer as dimeric monochloro-substituted p-xylylene.
Since the poly-p-xylylene coating is chemically stable and less susceptible to damage in an environment where the coating is exposed, the coating maintains constant insulating properties. Also, since the poly-p-xylylene coating is formed at room temperature by vapor phase epitaxy, it is possible to form an uniform insulating coating of the poly-p-xylylene over a substrate whose properties are degraded by heat or whose surface has a complex shape, without thermally damaging the substrate.
However, when the poly-p-xylylene coating is in use as the insulating coating for electrodes in ink chambers of an ink-jet printhead, there occurs a problem as described below.
Although it is possible to form an uniform coating of the poly-p-xylylene in ink chambers of ink-jet printheads having a complex shape, piezoelectric materials such as PZT used in the ink-jet printheads are sintered ceramics, and surfaces on which electrodes are to be formed attains a pear-skin finish with microscopic concavities and convexities because ceramic particles fall out of the surfaces when grooves are cut in there. When a parylene coating is formed over such a pear-skin-finished base, macroscopically a uniform coating is obtained. However, the parylene coating grown with the concavities and convexities of the base reflected has microscopic flaws (pinholes).
Since aqueous ink is an electrolyte solution with a very high electrical conductance in comparison with oil-based ink, if there is a pinhole through an insulating coating separating an electrode and the aqueous ink in an ink chamber, the electrode is electrically conducted with another electrode in an adjacent ink chamber through the ink infiltrating through the pinhole, so that electrolyte corrosion of the electrodes occurs. This causes ink-jet printhead reliability problems such as fluctuations in ink-ejecting properties during operation of an ink-jet printhead and inferiority in ink ejection in the ink-jet printhead caused by breaking of electrode wires. These problems also occur in an organic insulating coating formed over another kind of substrate such as a semiconductor.
To solve the problems, Japanese Laid-open Patent Publication No. 2001-96754 discloses a method for improving insulating properties of a parylene coating, by which after the parylene coating is formed polyimide resin is electrodeposited selectively over a pinhole and then sintered at 80° C. for 24 hours. According to the method, however, equipment is required for the electrodeposition of polyimide resin, thereby increasing production costs. Also, it is necessary to sinter the polyimide resin for a long time, so that production throughput is decreased.
On the other hand, Japanese Laid-open Patent Publication H11-309856 discloses a method for improving insulating properties of parylene coatings, by which coatings of two kinds of parylene having different structures are layered with plasma treatment performed to a lower parylene coating. According to the method, however, vacuum equipment is required for the plasma treatment, thereby increasing production costs.
An object of the present invention is to provide a method for producing an organic insulating coating which prevents electrolytic corrosion of electrodes by improving insulating properties of the organic insulating coating separating the electrodes from an electrolyte solution, as well as to provide an ink-jet printhead having stable ink-ejecting properties ensured by utilizing the method for producing the organic insulating coating.